1. Field of the Invention
This invention pertains to downhole drilling in general and more particularly to a rigid coupling having articulated connectors that transfer rotational force between two (2) axially misaligned shafts.
2. General Background
In downhole drilling, there are three (3) fundamental elements: a power generating motor section, a connecting rod section, and a bearing section. This invention pertains to the middle or connecting rod section.
There exists a variety of connecting rod sections that are currently in use. Their main purpose is to transfer, with as little loss as possible, the axial and rotational forces developed in the motor section to the bearing section. This is more complicated than it might at first appear because the transfer point of the motor section is usually a helical rod that not only rotates, but also gyrates with respect to the elongated axis of the well. The bearing section, on the other hand, cannot tolerate any gyration and must rotate exactly around a specific axis if damage to this bearing section is to be prevented.
In the past, U-joints were utilized to transfer the forces to the bearing section, but because of the tremendous pressures involved and the abrasive particles in the drilling mud, the pivoting or hinge pins soon wore out. Also, it was discovered that even though these hinge pins might be sealed, the U-joints were not able to withstand the torsion and axial loads applied.
An improvement over the U-joint was a "flexible rod" connection. Essentially, the connector was a small diameter elongated rod that connected between the gyrating motor section and the "fixed" bearing section. The gyration of the motor section was absorbed by the bending or flexing of the connector so that only a rotational force was delivered to the bearing section. A drawback to this "flexible rod", however, is the fact that continued flexing of the rod soon resulted in metal fatigue. Also, the rod was subject to friction and wear as it was moved around the interior of the well casing. Finally, the amount of axial thrust that could be applied to the bearing section was limited because the flexible rod "gave."
It is thus an object of this invention to provide a connecting rod section that can transmit the rotational force from a gyrating motor section to a nongyrating bearing section. Another object of this invention is to provide a connecting rod that is rigid or stiff and thus does not have the negative attributes of a "flexible rod" connection. A further object of this invention is to provide a means of accommodating a gyrating motor shaft without the drawback of a U-joint. Yet another object of this invention is to provide a connecting rod that is shorter than those currently used, thereby reducing the magnification of the bending moment forces on the connecting rod due to the off-set or misaligned gyration of the motor section. Yet another object of this invention is to provide a connecting rod that has an increased cross-sectional area in order to increase both its working life and its working strength. Still another object of this invention is to provide a connecting rod that is of monolithic construction so that it is easier to inspect for cracks or imperfections than is possible for other types of connecting rods which incorporate an outer covering or wear surface. Still another object of this invention is to provide a connecting rod that is quick and easy to assemble or disassemble, as the case may be, and attach or disattach to the motor section. Furthermore, an object of this invention is to provide a connecting rod that incorporates a superior method of providing and preserving the integrity of the packing which holds in the lubricant so as to enhance the life of the connecting rod. Another object of this invention is to provide a connecting rod that is relatively easy and inexpensive to construct, and one that does not require exotic materials or unique manufacturing methods. Yet another object of this invention is to provide a connecting rod that transfers mechanical motion smoother as compared to other connecting rods thus reducing vibration and enhancing the overall life of the tool. These and other objects will become obvious upon further investigation.